1. Field of the Invention
The present invention relates to a braking control device having an assist hydraulic pressure control section that adds an assist hydraulic pressure to a master cylinder hydraulic pressure. The pressure is added on the application of a brake operation force that exceeds the boost limit point at which the booster device becomes unable to amplify the brake operation force at the prescribed boost ratio.
2. Description of the Related Art
With the above type of braking control device, the master cylinder generates the master cylinder hydraulic pressure as a result of the force produced as a result of the brake operation force being amplified by a booster device, and the master cylinder hydraulic pressure is applied to each wheel cylinder. However, when the applied brake operation force exceeds the boost limit point at which the booster device becomes unable to amplify the brake operation force at the prescribed boost ratio, the master cylinder hydraulic pressure is insufficient relative to the target wheel cylinder pressure.
Thus, when a brake operation force that exceeds the boost limit point is applied, the assist hydraulic pressure control section calculates an assist hydraulic pressure to be added to the master cylinder hydraulic pressure based on data detected by a brake operation force detection sensor, and the assist hydraulic pressure is added to the master cylinder hydraulic pressure (e.g., refer to patent document 1).
FIG. 7 shows a relationship between a brake operation force F and a wheel cylinder pressure PWC for a conventional braking control device. The change in a master cylinder hydraulic pressure PMC when a brake operation force that exceeds the boost limit point T1 is applied is represented by a dotted line in the drawing.
When a brake operation force that has not reached the boost limit point T1 is applied, the assist hydraulic pressure is not added. At this time, the relationship between the brake operation force and the wheel cylinder pressure has a first correlation S1 when the brake operation force is increasing, a second correlation S2 after hysteresis is applied when the brake operation force is decreasing, and an intermediate correlation S3 that connects between the first correlation S1 and second correlation S2. The intermediate correlation S3, for example, is configured so that the wheel cylinder pressure decreases more gradually than in the first correlation S1.
A configuration is produced whereby, when the brake operation force decreases during the first correlation S1, the wheel cylinder pressure decreases in the same relationship as the relationship of the decrease in wheel cylinder pressure with respect to the decrease in the brake operation amount in the intermediate correlation S3, whereupon the wheel cylinder pressure decreases based on the second correlation S2. A configuration is produced whereby, when the brake operation force increases during the second correlation S2, the wheel cylinder pressure is kept constant, whereupon the wheel cylinder pressure increases based on the first correlation S1.
When a brake operation force that exceeds the boost limit point T1 is applied, the assist hydraulic pressure control section first determines the target the wheel cylinder pressure PWC, calculates the assist hydraulic pressure PA based on the difference between the wheel cylinder pressure PWC and the master cylinder hydraulic pressure PMC that has been determined, and then adds this calculated assist hydraulic pressure PA. As regards control of the assist hydraulic pressure, when the brake operation force is increasing, the assist hydraulic pressure control section controls the assist hydraulic pressure based on a first correlation extension section S1a produced by an extension of the first correlation S1 so that the relationship between the brake operation force and the wheel cylinder pressure is kept in the first correlation S1. In addition, when the brake operation force is decreasing, as when the brake operation force is increasing, the assist hydraulic pressure control section controls the assist hydraulic pressure based on the first correlation extension section S1a produced by extension of the first correlation S1 so that the relationship between the brake operation force and the wheel cylinder pressure is kept in the first correlation S1.
[Patent document 1] JP (Kokai) 2006-192945
With the above conventional braking control device, the relationship between the brake operation force and the wheel cylinder pressure when the brake operation force switches from increasing to decreasing is different from when a brake operation force that exceeds the boost limit point T1 is applied when a brake operation force that is below the boost limit point T1 is applied. When a brake operation force that exceeds the boost limit point T1 is applied, the first correlation extension section S1a applies, whereas when a brake operation force that is below the boost limit point T1 is applied, the intermediate correlation S3 applies, with the wheel cylinder pressure decreasing more gradually than in the first correlation S1. Consequently, when the brake operation force switches from increasing to decreasing due to a fluctuation in the brake operation force or another reason, the wheel cylinder pressure varies more when a brake operation force that exceeds the boost limit point T1 is applied than when a brake operation force that is below the boost limit point T1 is applied. For this reason, when a brake operation force that exceeds the boost limit point T1 is applied, the wheel cylinder pressure tends to change more quickly with respect to changes in the brake operation force, and the firmness with respect to changes in brake operation force may be insufficient.
In addition, with conventional braking control devices, when the brake operation force decreases in a situation where a brake operation force exceeding the boost limit point T1 is applied, the relationship between the brake operation force and the wheel cylinder pressure will adhere to the first correlation extension section S1a until the assist hydraulic pressure is released, to the intermediate correlation S3 after the assist hydraulic pressure has been released, and to the second correlation S2 thereafter. Consequently, the relationship of the decrease in wheel cylinder pressure with respect to the decrease in brake operation force will change before and after the assist hydraulic pressure is released, and will also change after the assist hydraulic pressure has been released, which causes significant alteration. For this reason, the operator may experience an unpleasant sensation as a result of the significant changes in deceleration rate.